Add logging for work items being processed.

lib/compiler/implementation/dart_backend/backend.dart

Index: lib/compiler/implementation/dart_backend/backend.dart
diff --git a/lib/compiler/implementation/dart_backend/backend.dart b/lib/compiler/implementation/dart_backend/backend.dart
index a7beccf9b31163e01aa369a16265ca5726af570a..9c8b5f7124991420e44a90332d144df98195b46b 100644
--- a/lib/compiler/implementation/dart_backend/backend.dart
+++ b/lib/compiler/implementation/dart_backend/backend.dart
@@ -91,6 +91,7 @@ class DartBackend extends Backend {
 
   String codegen(WorkItem work) {
     // Traverse AST to populate sets of reachable classes and functions.
+    log('codegen(${work.element})');
     FunctionExpression function = work.element.parseNode(compiler);
     function.body.accept(new TraversingVisitor(reachabilityVisitor));
   }
@@ -99,4 +100,6 @@ class DartBackend extends Backend {
   void assembleProgram() {
     compiler.assembledCode = '';
   }
+
+  log(String message) => compiler.log('[DartBackend] $message');
 }